Many integrated circuit devices include digital circuitry formed of metal-oxide-semiconductor (MOS) transistor devices, which are built using complementary MOS (CMOS) fabrication processes optimized for high-density, high-speed N-channel and P-channel MOS transistors. Such high-density circuitry is common in modern consumer electronic products such as wireless communications devices, portable computers, etc., in which digital circuitry is powered by batteries.
Many devices require MOS devices operable for low voltage applications and high voltage applications. For example, logic operations typically employ low voltage MOS devices, for example about 1.8 V where as power operations typically employ high voltage MOS devices, for example greater than 6 V. MOS devices for low voltage and high voltage applications can be and often are implemented on a single die or integrated circuit in order to conserve space and cost of fabrication.
A type of MOS transistor device employed in semiconductor devices is an N or P channel drain-extended metal-oxide-semiconductor (DEMOS) transistor device. The DEMOS devices are often employed for applications such as power switching circuits. The DEMOS devices employ a drain extension region which substantially increases operating voltages for the devices. Some examples of DEMOS devices include lateral diffused MOS (LDMOS) devices, REduced SURface Field (RESURF) transistors, and the like. DEMOS devices advantageously combine short-channel operation with high current handling capabilities, relatively low drain-to-source on-state resistance (Rdson), and the ability to withstand relatively high drain-to-source voltages without suffering voltage breakdown failure, where DEMOS device designs often involve a tradeoff between breakdown voltage (BVdss) and Rdson. In addition to performance advantages, DEMOS device fabrication is relatively easy to integrate into CMOS process flows, facilitating use in devices where logic, low power analog, or other circuitry is also to be fabricated in a single integrated circuit (IC).
A class of DEMOS transistor devices typically employed in high voltage applications are high voltage MOS (HVMOS) transistor devices. HVMOS devices include a thicker dielectric layer and a back gate region in addition to the drain extension region. HVMOS devices can be fabricated with low voltage CMOS devices and can use the N and P wells of the low voltage CMOS devices as back gate regions and/or drain extension regions. This can save space and cost during fabrication, but can also result in HVMOS devices having varied channel lengths. Threshold voltages for HVMOS devices are generally a function of channel length, so this also results in HVMOS devices having varied threshold voltages. The varied threshold voltages can cause memory operations, such as programming and reading, to be difficult to implement.